This invention relates to the feeding of articles and in particular to the feeding of mail items in franking machines.
In known franking machines, printing of the franking impression on a mail item is effected by means of a print drum carrying a printing plate for printing the invariable part of the impression and printing wheels settable to print a desired value of postage charge and the date. Franking of the mail item is effected during a single revolution of the print drum. During this rotation of the print drum, the mail item is pressed into printing engagement with the printing plate and printing wheels by means of an impression roller. To provide the required pressure for effecting printing, the impression roller is urged resiliently toward the print drum. After completion of printing of the franking impression the impression roller is retracted to allow the mail item to be ejected from the franking machine. Retraction of the impression roller is accomplished by a follower on a mounting for the impression roller engaging a cam rotating with the print drum. The cam is shaped such that the impression roller is free to be resiliently urged toward the print drum during the period of rotation of the drum in which the printing plate and printing wheels are in printing engagement with the mail item and is retracted by the cam away from the print drum during at least a part of the remainder of the revolution of the print drum.
Due to the shaping of the cam to allow the impression roller to move toward the print drum prior to the start of printing, the force of engagement of the follower on the cam tends to apply an additional forward driving force to the print drum during the period that the impression roller moves towards the print drum with the result that the drum is subjected to a varying rotational drive force during each revolution thereof.
Franking machines are required to print franking impressions on mail items of various thickness. For example mail items may vary in thickness from the relatively small thickness of labels for attachment to mail packets up to substantially thicker items consisting of envelopes containing a number of sheets of paper. In order to provide adequate printing pressure to the thinnest of mail items, the impression roller must be free to exert pressure when it is separated from the print drum by only the thickness of a label while permitting the leading edge of a relatively thick mail item to enter the nip between the print drum and impression roller.